[Rcpp-devel] Package Development with Rcpp, RcppArmadillo, BH and bigmemory for systems without full C++ extensions

[Dirk Eddelbuettel]

Hi Scott,

On 30 July 2014 at 11:53, Scott Ritchie wrote:
| I've been developing an Rcpp/RcppArmadillo package that makes use of BigMatrix
| objects from the bigmemory package, and I've had a series of confusing
| encounters with C++11, so I want to check that I'm doing the right thing.
| 
| Before using RcppArmadillo, I had enabled C++11 extensions by including
| SystemRequires: C++11 in my DESCRIPTION file, in order to turn off compiler
| warnings due to the BigMatrix class's use of boost headers.

That is one of several ways to turn it on.  

I prefer to use src/Makevars{,win} with a setting of

   CXX_STD = CXX11

as there may be other things to toggle in src/Makevars{,.win} (see below),
and to me it is more logical to keep the setting (C++11) along with the files
(in src/) -- but at the end of the day either way works.
 
| Now that I've added RcppArmadillo, I've had to use custom Makevars and
| Makevars.win files to correctly link the package to LAPACK libraries if they're
| installed. I've generated these files using RcppArmadillo.package.skeleton().
| 
| Now when I try to install my package on a different system (Ubuntu 12.04), I
| get the following warnings from files which include the RcppArmadillo.h header
| file:
| 
| 
|     WARNING: your C++ compiler is in C++11 mode, but it has incomplete support
|     for C++11 features; if something breaks, you get to keep all the pieces
| 
| 
| This is because the g++ compiler is version 4.6.3, so it only has limited
| support for C++11 extensions (-std=c++0x).

Correct.  Ubuntu 12.04 is old. 

If you can, upgrade to 14.04. 

If you can't, suffer the consequences such as an old compiler.  [ You could
also get a new g++ from another PPA, but I'd just upgrade. But maybe you
can't. ]
 
| Since I haven't written any code that makes use of C++11 extensions, I have
| tried turning them off in my RcppArmadillo files by adding #define
| ARMA_DONT_USE_CXX11 before the include statement #include <RcppArmadillo.h>.
| However, this does not get added to RcppExports.cpp so I still get the compiler
| warning once when I install the package.

If you do not use C++11, there may be another option -- provided you do not
plan to upload to CRAN.  CRAN is very black and white: C++ is either done by
the old 1998 standard, or the 2011 standard.

You can get 'long long' as I recall with intermediate value. 

Here is Section 2.2 of the g++ documentation:

  2.2 C++ language
  ================

  GCC supports the original ISO C++ standard (1998) and contains
  experimental support for the second ISO C++ standard (2011).
  
   The original ISO C++ standard was published as the ISO standard
  (ISO/IEC 14882:1998) and amended by a Technical Corrigenda published in
  2003 (ISO/IEC 14882:2003).  These standards are referred to as C++98 and
  C++03, respectively.  GCC implements the majority of C++98 ('export' is
  a notable exception) and most of the changes in C++03.  To select this
  standard in GCC, use one of the options '-ansi', '-std=c++98', or
  '-std=c++03'; to obtain all the diagnostics required by the standard,
  you should also specify '-pedantic' (or '-pedantic-errors' if you want
  them to be errors rather than warnings).
  
   A revised ISO C++ standard was published in 2011 as ISO/IEC 14882:2011,
  and is referred to as C++11; before its publication it was commonly
  referred to as C++0x.  C++11 contains several changes to the C++
  language, most of which have been implemented in an experimental C++11
  mode in GCC.  For information regarding the C++11 features available in
  the experimental C++11 mode, see
  <http://gcc.gnu.org/projects/cxx0x.html>.  To select this standard in
  GCC, use the option '-std=c++11'; to obtain all the diagnostics required
  by the standard, you should also specify '-pedantic' (or
  '-pedantic-errors' if you want them to be errors rather than warnings).
  
   More information about the C++ standards is available on the ISO C++
  committee's web site at <http://www.open-std.org/jtc1/sc22/wg21/>.
  
   By default, GCC provides some extensions to the C++ language; *Note
  Options Controlling C++ Dialect: C++ Dialect Options.  Use of the '-std'
  option listed above will disable these extensions.  You may also select
  an extended version of the C++ language explicitly with '-std=gnu++98'
  (for C++98 with GNU extensions) or '-std=gnu++11' (for C++11 with GNU
  extensions).  The default, if no C++ language dialect options are given,
  is '-std=gnu++98'.


So if you are aiming for local / in-house use, then

   PKG_CXXFLAGS=-std=c++03

may be all you need.  It's been a while since I tried as I keep most of my
packages in a 'ready for CRAN' mode, and tend to keep those local to work in
a similar shape.  
 
| I've additionally removed SystemRequirements: C++11 from my DESCRIPTION file
| but that seems to make no difference (my Rcpp code that doesnt use the
| armadillo headers still compiles without warnings, despite including the BH
| dependency.

Well everything builds. It is just a warning, or set of warnings.
 
| I'm not sure how much of an issue this is, because none of my code seems to be
| broken when I install on the system without full C++11 extension support. Can I
| just ignore this warning, and is what I have done correct?

Yes of course. Lots of packages install with a ton of warnings when you
default "-Wall -pedantic" as I do. I think RJSONIO is just one example that I
upgraded in the last few days.  

Warnings != Errors but it is still best to not have any as they may mask errors.

Dirk

 
| My package versions on both systems are:
| 
|     Rcpp_0.11.2
|     RcppArmadillo_0.4.320.0
| 
| 
| Kind Regards,
| 
| Scott Ritchie
| 
| 
| 
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